In cellular networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the cellular network is deployed.
In a cellular network where users in a cell are served by network nodes there may be areas with high traffic, i.e. high concentration of users. By a user is here meant a wireless user device, such as a user equipment, UE. In such areas it is desirable to deploy additional capacity to ensure user satisfaction. The added capacity may be in the form of additional macro network nodes or network nodes with lower output power, such as micro network nodes or pico network nodes covering a particular area, thus concentrating the capacity boost to the particular area.
In the cellular network there may also be particular areas with coverage below a predetermined quality criterion where there thus may be a need for coverage extension. One way to mitigate this problem is to deploy a network node with low output power to concentrate the coverage boost to the particular areas. In general terms, a network node in the form of a so-called macro base station provides a wide area coverage (also called a macro cell). In the coverage area of the macro base station also low power network nodes may be deployed to provide small area capacity/coverage. Examples of such low power network nodes are so-called pico base stations, relays and home base stations (femto cells).
One argument for choosing network nodes with lower output power in the above scenarios is that the impact on the network as a whole may be minimized, e.g. for example concentrating any potential added interference of the network to the particular areas in which the additional network nodes with lower output power are deployed.
Additionally, there is currently a common drive in the direction towards the use of low power network nodes. The different terms used for these types of network deployments are Heterogeneous networks, multilayer networks or shortly HetNets.
Since cells generally are operated with different pilot power levels, there may be imbalances between uplink and downlink in the network. One reason is that cells to serve the UE are typically by the network nodes selected based on received signal strength. This implies that a UE is served by the best downlink cell alternative (according to some quality of service or throughput criterion). However, the uplink channel quality depends mainly on the distance between the UE and the serving network node, thus being independent of the pilot power. This means that for cell selection based on downlink pilot signaling, a UE may have a better (according to some quality of service or throughput criterion) uplink to a non-serving network node. In such case a different solution called Cell Range Extension (CRE) may be used. In general terms, CRE may be regarded as a cell relation specific offset that the UE considers in the report triggering condition evaluations. One effect is that small cells are expanded into a CRE area, where the downlink from the macro base station is more favourable than the downlink from the pico base station.
Once such cells are detected by the UE and reported to the macro base station, the macro base station may decide to handover the UE to the detected pico base station. Such handover might be preceded by allocation of so-called Almost Blank Subframes (ABS) by the the macro base station (see the 3GPP standardization documents TS 36.331 and TS 36.423). ABSs are so-called “protected subframes”. In such subframes the macro base station limits its transmission. Therefore, a small cell neighbouring the macro base station will experience reduced interference during transmission of ABS subframes.
Once the UE is handed over to the CRE of the small cell, the thus serving pico base station may decide to serve the UE during ABSs, due to the otherwise high downlink interference the UE would experience from the macro base station. Further, the UE should be configured by the pico base station so as to measure neighbouring cells on ABSs. This may ensure that the measurements are not impacted by high levels of downlink interference from the macro base station.
Advanced load sharing mechanisms, such as the process summarized above, may be beneficial to avoid overload in cells by distributing load between frequencies, radio access technologies and radio network nodes. However, implementing such advanced load sharing may require more computational capacity and more and measurement reporting capacity than is available in radio access control circuitries in current network nodes. Moreover, such advanced mechanisms are challenging to effectively combine with further coordinated resource management.
Hence, there is still a need for an improved load sharing in cellular networks.